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Reclassifications out of accumulated other comprehensive (loss) (Details 16) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,184,423 | $ 1,285,782 | $ 1,060,595 | $ 931,604 | $ 1,081,144 | $ 1,173,518 | $ 967,962 | $ 852,807 | $ 4,462,404 | $ 4,075,431 | $ 4,050,492
Interest expense |  |  |  |  |  |  |  |  | (83,917) | (76,699) | (81,354)
Earnings (loss) from equity method investments |  |  |  |  |  |  |  |  | (132) | 5,383 | 4,693
Income before income taxes |  |  |  |  |  |  |  |  | 415,618 | (45,467) | 336,226
Income taxes |  |  |  |  |  |  |  |  | 136,736 | (31,146) | 110,274
Net income (loss) |  |  |  |  |  |  |  |  | 278,570 | (754) | 213,026
Reclassification adjustment for gain (loss) on derivative instruments included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Income before income taxes |  |  |  |  |  |  |  |  | 6,737 |  |
Income taxes |  |  |  |  |  |  |  |  | (2,548) |  |
Net income (loss) |  |  |  |  |  |  |  |  | 4,189 |  |
Amortization of postretirement liability losses included in net periodic benefit cost |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Net periodic benefit cost |  |  |  |  |  |  |  |  | (3,277) |  |
Income taxes |  |  |  |  |  |  |  |  | 1,276 |  |
Net income (loss) |  |  |  |  |  |  |  |  | (2,001) |  |
Reclassification adjustment for loss on foreign currency translation adjustment included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Earnings (loss) from equity method investments |  |  |  |  |  |  |  |  | (213) |  |
Earnings (loss) from equity method investments, tax |  |  |  |  |  |  |  |  | 70 |  |
Net income (loss) |  |  |  |  |  |  |  |  | (143) |  |
Reclassification adjustment for loss on available-for-sale investments included in net income [Member] |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Other income |  |  |  |  |  |  |  |  | (168) |  |
Income taxes |  |  |  |  |  |  |  |  | 59 |  |
Net income (loss) |  |  |  |  |  |  |  |  | (109) |  |
Total reclassifications |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  |  |  |  |  | (1,936) |  |
Commodity derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 7,803 |  |
Interest rate derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  | $ (1,066) |  |

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